Saudi Arabia is now prioritising its own long-term market share over short/medium-term revenue maxmisation.

Oil production costs in US > those in OPEC countries, especially Saudi Arabia, so low oil price is bad for US oil (supply less likely to expand) and hence allows Saudi Arabia to maintain (or even increase) its market share.

Saudi has built up huge excess capacity and has a very low extraction rate, hence has the ability to go it alone despite the wishes of other major OPEC members like Iran and other oil exporters like Russia (lower oil prices limit what Saudi believes are Russia and Iran's destructive geopolitical activities in the Middle East).